Global warming pushing rivers to lose their ice cover sooner, leading to more greenhouse gas emissions
Frozen rivers also support important transportation networks for communities and industries located at high latitudes
If global warming continues unchecked, the Earth could risk losing ice cover on rivers in the Rocky Mountains and northeastern USA. Every year, rivers in these regions could experience ice loss six days earlier than usual for every one degree Celsius increase in global temperatures, finds a study.
"The observed decline in river ice is likely to continue with predicted global warming," the study explains. This decline will have economic and environmental consequences, say scientists. This is because frozen rivers enable transportation for communities and industries located at high latitudes and near the poles.
"We detected widespread declines in monthly river ice coverage," says Xiao Yang, a postdoctoral scholar in the UNC-Chapel Hill geological sciences department and lead author of the study. “And the predicted trend of future ice loss is likely to lead to economic challenges for people and industries along these rivers and shifting seasonal patterns in greenhouse gas emissions from the ice-affected rivers,” says Yang.
More than half of Earth's rivers freeze every year during the cold season. Ice along rivers is important in controlling the levels of greenhouse gases in the environment, especially since ice sheets store carbon dioxide — a notorious greenhouse gas. In doing so, ice sheets regulate the amount of greenhouse gases released from rivers into Earth's atmosphere.
But higher temperatures mean rivers will lose their ice cover sooner. Without ice cover, rivers could end up emitting a lot of carbon dioxide into the atmosphere. Further, this could warm water bodies and harm fish and other aquatic organisms.
An earlier study has recorded vanishing ice cover on lakes in the Northern Hemisphere. Researchers found that thousands of lakes, situated across Wisconsin to Japan, that used to once freeze reliably every winter, are not freezing anymore. These lakes are already seeing some years without ice, they add. At this rate, experts predict, an extensive loss of lake ice will occur within the next generation.
In the current study, Yang and his colleagues relied on satellite images to look at freezing rivers of the past to predict future changes in river ice. "We used more than 400,000 satellite images taken over 34 years to measure which rivers seasonally freeze across the world, which is about 56% of all large rivers," says Yang.
To make sense of the ice cover we have lost in the recent past, the team compared ice cover changes in the present (2008-2018) to that of the past (1984-1994). They found a monthly decline ranging from 0.3% to 4.3% points, globally, with the greatest declines happening in the Tibetan Plateau, eastern Europe and Alaska.
As for the future, the team compared expected river ice cover from 2009 to 2029 and from 2080 to 2100. Results showed that the Northern Hemisphere could experience monthly declines, ranging from 9% to 15% in the winter months and 12% to 68% during the spring and fall. The Rocky Mountains, northeastern United States, eastern Europe and the Tibetan Plateau are expected to take the heaviest impact.
"Ultimately, what this study shows is the power of combining massive amounts of satellite imagery with climate models to help better project how our planet will change," says Tamlin Pavelsky, UNC-Chapel Hill Associate Professor of global hydrology.
The study has been published in Nature.